As of now, this is what happens when we "paste" images greater than the size of the reply "box":

(part of the image is hidden)

Inspecting the CSS, there's this piece: .reply-body img in which the max-width is set as inherit. If we disable this, the .fr-view img will come into action and the max-width will be 100%, leading to this:

The image now fits and, if anyone wants to see the full image, they can do it.